Bronze Framing for Historic Stained Glass: A New Case Study from the York Glaziers Trust
The prevention of ongoing deterioration of historic stained glass through the provision of effective and ventilated protective glazing has been widely researched and published. In the most commonly employed system, for which the most extensive research data are available, the historic glass is moved inside the building, supported in a frame, with new outer glazing positioned in the original glazing groove, allowing the interspace between to be ventilated from inside the building. Very little appears in print however on the materials and methodologies used in the manufacture of these systems, and in particular the methods used for the essential support of the stained glass, now positioned in front of the original glazing groove. It would seem that the framing methods are deemed so commonplace that publication is thought unnecessary, although in all cases with which this author is familiar, the assumption is that a non-ferrous metal alloy should be used as the framing material. The following article describes a case study, of the approach to the protection and framing of the Great East Window [Fig. 1] of York Minster by the York Glaziers Trust, suggesting an alternative method to that most commonly applied for the purpose across Europe. It is hoped that these methods will provoke interest and discussion amongst fellow practitioners and architects in the field of stained glass conservation alike.
By the end of 2015, the first phase of an extensive stained-glass conservation campaign by the York Glaziers Trust, for the Great East Window (c.1405–08) of York Minster, will be complete. Eighty-one panels arranged across nine rows of this nine-light window depict the Apocalypse, derived from the Book of Revelation of St John (rows 2–11), with nine panels depicting historical figures of kings, popes and archbishops (row 1), and 18 canopy heads (rows 6 and 12). Following conservation, they will be returned with a new protective-glazing system, designed by Nick Teed, Senior Conservator at YGT, in collaboration with York Minster Surveyor to the Fabric, Andrew Arrol. The designs were informed by trial installations of a number of protective-glazing systems that were environmentally monitored over a twelve-month period between 2007 and 2008, with comprehensive analysis of the data and the benefits of each trial system provided by Dr Ivo Rauch. During this complex conservation project, new methods have been developed at the YGT, for the fabrication of the Great East Window’s bespoke protection system. This article will focus in detail on the design and manufacture of the bronze frames used for the stained glass panels in this system.
Any protective-glazing system should be designed specifically for the given needs of each individual window, and be sympathetic to the surrounding materials (in the case of the Great East Window, historic carved stone). The Great East Window has the distinction of having been one of the first historic windows to be protected in England. Plate glass installed in an external frame in 1861 as protection against ‘the products of combustion’ was replaced in the early 20th century with plain quarry glazing set onto iron T-bars, and an additional set of T-bars was installed to support the stained-glass panels. Both sets occupied the space in the central portion of the stone mullions originally intended by the medieval masons to receive the glazing. The size of the T-bars necessitated that new grooves be cut into the stone, either side of the central (original) glazing groove, to accommodate the protective glass and stained glass. Both layers of glass in this system were fully pointed with mortar, without the provision of ventilation to the inside or outside of the building. The new protective-glazing system aimed to maintain the 19th-century positions of both stained glass and protective glazing, but to replace the ferrous materials used in the 20th century with a more durable alternative, at the same time introducing effective ventilation to the interspace from the interior of the building. It has been possible to keep both the protective glazing and the stained glass within the central ‘glazing position’ of the mullions, as opposed to the commonly used system of mounting the framed stained glass to the inner cavetto profile of the stonework.
Each panel of either stained glass or protective glass in the lancets of the Great East Window will be set onto bronze T-bars and held with pins made of the same metal composition. The protective outer glazing is being manufactured from Lamberts’ ground-breaking Restauro UV© glass (Fig. 2). This is the first use in the United Kingdom of this product. In addition to the long-term protection to light-sensitive organic conservation materials, such as the epoxy resins used in the bonding of fractured glass, that it provides, there is the inherent beauty of this mouth-blown glass, when viewed from the exterior of the building, to consider.
The choice of architectural bronze for the installation of the supporting structures of the window was a natural one. Often referred to as ‘manganese bronze’ due to the presence of small quantities of that element in its composition, this metal has long been chosen by stained-glass conservators and architects for its strength, appearance, machinability, and durability. The YGT chose McKechnie Brass’s bronze composition ‘Alloy 300’ for the manufacture of all sections of the U-channel for framing, support bars, T-bars, and fixing-plate material  including the rods from which the glazing pins were made). As an alloy that sits lower in the reactivity series than the more commonly used irons or steels, it represents a good alternative for the replacement of non-original ferrous materials. The decision to manufacture all elements of the support system from the same (or closely related) material composition was made in order to minimize the risk of future deterioration through the bi-metallic corrosion associated with incompatible metals. Other metals used by conservators across Europe for the framing of stained-glass panels include brass or copper.
Usually, the placing of the protective glazing in the original glazing groove necessitates the support of the stained-glass panels in a frame. This brings the advantage that the stained-glass panels can be more easily removed for future maintenance or for security during renovations to surrounding materials. Traditionally stained-glass studios have made frames out of brass or bronze slim-line U-channel, soldered together with mitred corners, with integral support bars also soldered to the surrounding frame. If required, screw-fixing brackets to secure the frames into the masonry are attached to the frames with solder, or are riveted. These effective methods of manufacture have become widespread across Europe, and many historic windows have benefitted from the installation of such protective systems. The great size and height of the Great East Window meant that from the outset the YGT sought an alternative method of manufacture, which focused on increasing the strength of the individual frames and providing far greater ease of removal of the panel from its frame in the future. Given the high cost of materials, increasing the chances of re-use of the frame was also a consideration.
Solder works well in securing prepared brass or bronze, however, it only effectively provides an adhesive bond between two metal surfaces. To remove a panel from a soldered bronze frame requires heat, and significant effort, and is not without risk to the historic glass. The elements of the frame are often distorted and damaged in the process. With the help of a local engineering company, a number of alternative framing methods were empirically tested, with a particular focus on silver solder and brazing methods for the bronze framing angles. Both approaches require significantly more heat than the traditional soldering process, and use a blow-torch with combined oxygen and acetylene. For this reason the jointing cannot be made in the proximity of the stained-glass panel, as the risk of damage would be very high. Silver solder provided a very strongly bonded joint, which could not be broken by hand in a shear-strength test (unlike the traditional soldered joint). A brazed joint uses a rod of similar composition metal and far greater heat to effectively melt the two surfaces together. The brazed joint was even stronger, but was also not suitable for use in proximity to the stained glass, and both methods required significant cleaning and polishing of the metal frame section following the treatment. With both methods the high heat required to make the joints mean that the frame cannot be manufactured with the stained glass within it, requiring that some pressure be exerted to introduce the stained glass into the partially constructed frame and necessitates at least an alternative method for securing the top corner joints of the frames.
The problems associated with these ‘hot working’ methods of assembly led to the investigation of a further approach based on simple screw fixings to hold the frames together. Mechanical screw fixings are very strong and once made, the component parts are easy to assemble and disassemble as necessary. Using 14mm x 2mm U-channel, 6mm x 12mm support bar, and 3mm flat bronze plate a prototype frame was made. Corner fixing brackets were made using a band-saw, and M6 brass screws were prepared and cut to an appropriate size [Fig. 3]. A discussion with McKechnie Brass over the possibility of manufacturing ‘Alloy 300’ screws was initiated, but the firm assured us that brass screws were sufficiently close in compatibility with the bronze to avoid future problems. All fixing points were drilled and tap threaded. Despite the slim 2mm profile of the bronze section we were astonished at the clean thread cut into the material by the tapping tool and the strength of the joint once screwed together.
The only disadvantage of the system at this stage appeared to be the time taken to manufacture the fixing plates; this problem was solved however by the decision to have these components made in quantity by a metals fabricator, a considerable saving of conservator’s time. A further development was made following the decision to set the 6mm x 12mm support bars on edge in order to increase the strength of the overall frame [Fig. 4]. This necessitates an additional fixing bracket in order to screw the bar to the frame section.
The only use of solder in the framing process [Fig. 5] is in the attachment of a lead strip on the back edge of each frame in order to eliminate any daylight where the frame meets the side of the window opening. The original intention of clamping the lead strip into position was rethought following a conversation with an archaeological metals conservator. Bronze corrosion is a common problem in the conservation of Greek and Roman statues and one method of prevention is to install a sacrificial lead anode, the purpose of which is to bear the brunt of any potential future corrosion that may occur. Whilst installation in an internal protected environment means that conditions are unlikely to be sufficiently damp to induce any form of galvanic corrosion, it is reassuring to think that the soldered lead strip may fulfil the role of a sacrificial lead anode and reduce or even eliminate the risk of potential future damage to the historic panel. While these thoughts are a little speculative at present, it is hoped that further research on this subject may prove to be fruitful.
Whilst each of the rectangular panels will be set onto bronze T-bars [Fig. 6] held in place with pins , the tracery panels have no such support bar and require screw fixings into the surrounding masonry. The complicated shapes of the Great East Window tracery panels, necessitate the bending of the framing section. For this a 9mm x 2mm U-channel is used, and the forms are made precisely using a specially adapted tool made by J. & C. R. Wood Ltd in Hull, under the brand name Metal-Craft. Each junction of a shaped panel is screw-fixed together with M4 machine screws and specially made fixing plates. The screw-fixings are made from extremely durable phosphor bronze strip. Slots milled into the front and back faces of the U-channel allow the bronze fixing-strip to be dropped in once folded, and then folded back around to the front side of the frame. In this method no solder or riveting is required, the fixings are remarkably strong and also offer a little flexibility to accommodate potential expansion and contraction of the frames [Fig. 7].
These methods of manufacture have now been tried and tested on the St Stephen Window (nII) of York Minster, reinstalled in 2014 as part of the York Minster Revealed project [Fig. 8]. The frames also provide very good support for panels in long-term storage, which require an easily reversible system. The method has been employed to great effect, for example, in the framing of a panel by Henry Gyles (dated 1670) for the York City Art Gallery [Figs 9 and 10]. Numerous technical adaptations can be made with such a system to suit different settings or contexts. Fixing bars and brackets can be made on the back face of the panel should a client wish to have a very clean look to the frame.
In summary, the system developed by the York Glaziers Trust offers simplicity, flexibility, strength, and true reversibility for the metal framing of historic stained-glass panels. Once the system and equipment required is in place the frames can be made speedily and offer a sustainable long-term solution to the support and protection of historic stained glass panels. It is hoped that the information on this system will be of benefit to the wider conservation community and promote further discussion on the critical choices to be made in the development of protective-glazing systems for historic stained glass.
Nick Teed, ACR
1. Stefan Oidtmann, Johanna Leissner, Hannelore Römich, ‘Schutzverglasungen’, in A. Wolff (ed.), Restaurierung und Konservierung historischer Glasmalereien, Mainz, 2000, pp. 167–209), English translation, ‘Protective Glazing’, http://www.cvma.ac.uk/conserv/glazing.html, accessed 23 February 2015; Adriana Bernardi et al., ‘Conservation of Stained Glass Windows with Protective Glazing: Main Results from the European VIDRIO Research Programme’, Journal of Cultural Heritage, v/14, 2013, pp. 527–36.
2. Described in Sarah Brown, Apocalypse: The Great East Window of York Minster, London, 2014.
3. Dr Ivo Rauch, ‘Climate Measurements for Three Windows in York Minster’, unpublished data report, March 2008.
4. Glashutte Lamberts, http://www.lamberts.de, accessed 23 February 2015.
5. U-channel refers to an extruded section of metal alloy which forms a framed surround for the stained glass panels offering support in a protective glazing installation.
6. None of the ferramenta or fixing materials being replaced in this project was medieval.
7. National Physical Laboratory, ‘Guides to Good Practice in Corrosion Control: Bimetallic Corrosion’, http://www.npl.co.uk/upload/pdf/bimetallic_20071105114556.pdf, accessed on 23 February 2015.